1. Field of the Invention
The present invention relates to a subscriber circuit which accommodates a public (coin) telephone set, and more particularly to a technique for moderating, when a public telephone set accepts a coin, the influence of the acceptance of the coil coin upon a voice signal.
2. Description of the Related Art
A subscriber circuit which accommodates a public telephone set reverses, as a signal when it accepts a coin thrown in by a call originating person in accordance with a telephone rate, the polarity of loop current slowly upon acceptance of the coin and then reverses, after the coin is confirmed, the polarity of loop current slowly again. Since a variation in voltage between two subscriber lines then is superposed with a voice signal which is transmitted similarly as a variation in voltage between the two subscriber lines, the system must be designed so that the voltage variation as a signal of acceptance of a coin may not disturb transmission of the voice signal.
FIG. 1 shows a basic construction of a conventional subscriber circuit. Referring to FIG. 1, the frequency band usually handled as a voice signal on a telephone circuit ranges from 300 Hz to 3,400 Hz, and a voltage variation of a frequency lower than 300 Hz which appears between subscriber lines 1 and 2 is suppressed principally by a channel filter 6.
However, usually a high-pass filter 5 for removing a high dc voltage applied to the subscriber lines to supply loop current is placed between a line voltage detection circuit 4 for detecting a voltage variation between the subscriber lines 1 and 2 and the channel filter 6. Here, designing for preventing such slow reversal of the polarity of loop current which is preformed by a loop current supply circuit 3 is performed, for example, in such a manner as described below.
The slowness of reversal of the polarity of loop current is first set linearly so that components of the voice band may be reduced to such a magnitude that they cannot be sensed as noise by a speaker. Although a slow voltage variation by such setting is attenuated by the high-pass filter 5, since the high-pass filter 5 is designed such that it does not have an overlapping function with the channel filter 6 and is not complicated, the attenuation amount is limited, and a slow voltage variation remains with the output of the high-pass filter 5. The sensitivity of the line voltage detection circuit 4 is set so that a sum voltage variation of the slow voltage variation and the voice signal may not exceed the range of linear operation of the channel filter 6.
Therefore, the voice signal introduced to the channel filter 6 decreases by an amount corresponding to the slow voltage variation remaining with the output of the high-pass filter 5. The voice signal is required to be amplified to a predetermined level by the channel filter 6, and noise generated by the line voltage detection circuit 4 and the high-pass filter 5 and input equivalent noise generated by the channel filter 6 must be low as much. Accordingly, with such a designing example as described above, various severe requirements for a noise characteristic must be satisfied.
It is an object of the present invention to provide a subscriber circuit for a public telephone set which eliminates the necessity to make a voice signal to be introduced to a channel filter a small signal.
In order to attain the object described above, according to the present invention, a slow voltage variation which remains with an output of a high-pass filter is estimated to produce an estimation signal, and the estimation signal is subtracted from an input signal to a channel filter.
In particular, according to the present invention, there is provided a subscriber circuit for a public telephone set, comprising a loop current supply circuit for supplying loop current between two subscriber lines, a line voltage detection circuit for detecting a voltage variation between the two subscriber lines, a high-pass filter for principally removing a high dc voltage from an output signal of the line voltage detection circuit, a channel filter for setting a voice signal band, variation signal generation means for generating a variation signal corresponding to a voltage variation applied between the two subscriber lines upon and caused by acceptance of a coin, and subtraction means for subtracting the variation signal from a signal of the voltage variation between the two subscriber lines detected by the line voltage detection circuit.
The variation signal generation means may include a voltage holding circuit for holding a voltage detected by the line voltage detection circuit prior to a reversal of the polarity of loop current, a reversal circuit for reversing the polarity of the voltage held by the voltage holding circuit, and a low-pass filter for buffering the voltage of the reversed polarity from the reversal circuit with a characteristic same as that of a low-pass filter which buffers the polarity reversal in the loop current supply circuit.
As an alternative, the variation signal generation means may include a voltage holding circuit for holding a voltage detected by the line voltage detection circuit prior to a reversal of the polarity of loop current, a reversal circuit for reversing the polarity of the voltage held by the voltage holding circuit, a high-pass filter for attenuating the voltage of the reversed polarity from the reversal circuit with a characteristic same as that of a high-pass filter for removing a high dc voltage, and a low-pass filter for buffering the output voltage of the high-pass filter with a characteristic same as that of a low-pass filter which buffers the polarity reversal in the loop current supply circuit.
As another alternative, the variation signal generation means may include a voltage holding circuit for holding a voltage detected by the line voltage detection circuit prior to a reversal of the polarity of loop current, a buffer amplifier to which the voltage held by the voltage holding circuit is inputted, a reversal amplifier for reversing the polarity of an output of the buffer amplifier, a pair of switches for extracting the outputs of the buffer amplifier and the reversal amplifier in response to a signal which indicates reversal of the polarity of loop current and another signal representing acceptance of a coin, respectively, and a low-pass filter for buffering the voltage extracted by the switches with a characteristic same as that of a low-pass filter which buffers the polarity reversal in the loop current supply circuit.
As a further alternative, the variation signal generation means may include a voltage holding circuit for holding a voltage detected by the line voltage detection circuit prior to a reversal of the polarity of loop current, a buffer amplifier to which the voltage held by the voltage holding circuit is inputted, a reversal amplifier for reversing the polarity of an output of the buffer amplifier, a pair of switches for extracting the outputs of the buffer amplifier and the reversal amplifier in response to a signal which indicates reversal of the polarity of loop current and another signal representing acceptance of a coin, respectively, a high-pass filter for attenuating the voltage extracted by one of the switches with a characteristic same as that of a high-pass filter for removing a high dc voltage, and a low-pass filter for buffering the output voltage of the high-pass filter with a characteristic same as that of a low-pass filter which buffers the polarity reversal in the loop current supply circuit.
In the subscriber circuit for a public telephone set, a signal which is equivalent to a voltage variation between the two subscriber lines upon acceptance of a coin from which a voice signal is removed is produced, and the signal is subtracted from a voltage detected from the two subscriber lines to extract only the voice signal from the voltage variation between the two subscriber lines. Consequently, the voice signal having a sufficiently great amplitude can be supplied to the channel filter in the following stage.